This invention relates in general to sewing machines and in particular to a new and useful drive device for positioning a sewing machine main shaft when its positioning motor drive is stopped, for example, to cut a thread.
In certain cases it is necessary to turn the sewing machine back, after having stopped it in a certain position, e.g. in needle-up or needle-down position, by a certain angle of the main shaft into another position. This applies for instance to chain stitch sewing machines where the shuttle has penetrated into the needle thread loop in the needle-up position and must be moved out of it to be able to remove the work unhindered at the end of a seam.
In backstitch sewing machines, the thread feeding lever of the needle bar lags by about 30.degree. behind the main shaft rotation, i.e. the needle bar, having passed through its highest position, is already moving downwardly while the thread lever is still moving upwardly. Because of this situation resulting from the sewing practice, it may be that when the sewing machine is stopped in needle-up position after the cutting of the thread and if the thread end is not clamped or retained, the thread is pulled out of the eye of the needle, as the sewing machine is started again, by the upwardly moving thread feeding lever, the start of sewing being thus delayed by the rethreading. For this reason often the thread lever up position is chosen as the stopping position after the after the cutting of the thread. But since the needle bar has then already moved downward by a certain distance, the passage space under the needle is, in many cases, too small to be able to remove or to insert the work unhindered. The seamstress must then grip the handwheel and turn the machine back by a certain angle to the needle-up position. This, however, is complicated and time consuming.
From German Pat. No. 910,023 a device for driving and stopping a sewing machine in predetermined positions is known where the stopping process occurs in that the sewing machine main shaft is first switched from full speed to a lower speed, at which it is advanced to a predetermined position and stopped there, whereupon it is automatically rotated back by a certain angle and again stopped. The change of speed and direction occurs directly at the drive motor. Such a drive is relatively complicated and expensive. Nor is it needed for all sewing machines and sewing processes.
Through German GM No. 81 24 815 a drive device to be retrofitted to sewing machines is known where, besides the V-belt pulley of the drive motor of the sewing machine, a driver disk is provided with a clutch disk. One of the device has a thread disposed on a threaded spindle adapted for rotation in the belt guard with adjustable tightness of motion. The other model has a cylindrical bearing bore and is arranged on a bearing pin which is also adapted for rotation in the belt guard with adjustable tightness of motion. A cross pin is inserted into the bearing pin and it protrudes into an oblique slot in the hub of the clutch disk. For rotating the clutch disk a pneumatic cylinder is used, whose piston rod engages at a crank arm of the clutch disk.
To turn the stopped sewing machine back into thread lever up position after the thread has been cut, compressed air is supplied to the pneumatic cylinder, whereby the clutch disk is rotated by a certain angle counter to the normal running direction of the motor shaft. In so doing, the one model of the clutch disk is displaced by the thread pitch, and the other model by the inclination of the oblique slot, at first axially into engagement with the drive disk and then, in the further course of the roration, the sewing machine is turned back into the needle-up position through the belt drive.
This device does indeed operate with a good stopping precision and can be uncoupled quickly enough for continued sewing, but it is quite complicated in construction and must be readjusted from time to time because of the abrasion at the clutch and driver disks.